The present invention relates to a solar cell.
In general, a solar cell has a semiconductor wafer of one conductivity type and a thin layer of conductivity type opposite to that of the semiconductor wafer. The thin layer is formed on the semiconductor wafer by doping or diffusing impurities into the semiconductor wafer to establish a P-N junction between the thin layer and the semiconductor wafer. Distortion of the crystal lattice is unavoidably formed in the thin diffusion layer when the impurities are diffused into the semiconductor wafer, which distortion acts as electron-hole recombination centers. Therefore, a great number of electron-hole pairs created by photons entering the diffusion thin layer are recombined at the said electron-hole recombination centers before they contribute to opto-electric conversion.
It will be clear that the opto-electric converting efficiency will be enhanced as the diffusion thin layer becomes thinner. But it is impossible to form the thin diffusion layer at a desired thickness, that is, for example, around 0.5 .mu.m, because the P-N junction will be damaged when an electrode is formed on the very thin diffusion layer through the use of conventional heat treatment techniques.
Accordingly, an object of the present invention is to provide a solar cell having a high opto-electric converting efficiency.
Another object of the present invention is to provide a solar cell sensitive to radiation having a very short wavelength.
Still another object of the present invention is to provide a solar cell having a thin diffusion layer suited for enhancing the optoelectric converting efficiency and preventing the P-N junction from being damaged.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objectives, pursuant to the present invention, a thin diffusion layer is formed in such a manner that the layer has a thickness of around 3 .mu.m at areas where an electrode is formed and has a thickness of around or below 0.5 .mu.m at regions where the electrode is not formed. With such an arrangement, radiation having a wavelength of about or shorter than 400 m.mu.m can be used for performing optoelectric generation and, moreover, the P-N junction will not be damaged since the thin diffusion layer is formed to have a sufficient thickness, namely, of around 3 .mu.m at the position where the electrode is formed.